1. Field of the Invention
The present invention relates to a magnetic composite material, and more particularly, to a magnetic composite material capable of realizing a magnetic refrigeration cycle using a magnetic field relatively easily produced by permanent magnets in a room temperature range and a method for producing the magnetic composite material.
2. Description of the Related Art
Recently, the technique for attaining magnetic refrigeration in a room temperature range has been aggressively studied. Magnetic refrigeration generates low temperatures as follows by applying the magnetocaloric effect. This effect is a phenomenon in which the temperature of a magnetic material changes when an external magnetic field is changed with respect to the magnetic material while the magnetic material is thermally insulated.
The entropy of a magnetic material is changed depending on whether a magnetic field is applied or not, owing to the difference between the degrees of freedom of the electron spin system. With this entropy change, the entropy transfers between the electron spin system and the lattice system. A magnetic refrigeration system uses a magnetic material having a large electron spin, and by taking advantage of this large entropy change between the spin system under magnetic field and the spin system without magnetic field, the magnetic refrigeration system generates low temperature.
In 1997, Zimm of the U.S.A. built a prototype of Active Magnetic Refrigeration apparatus (AMR) using a packed column filled with fine spherical Gd, and succeeded in a continuous steady-state operation of the magnetic refrigeration cycle at room temperature for one year or longer (C. Zimm et al., Advances in Cryogenic Engineering, Vol. 43 (1998), p.1759).
U.S. Pat. No. 5,743,095 described a Gd5(Ge,Si)4-based material, which is an intermetallic compound formed of gadolinium-germanium-silicon, as a magnetic material from which a very large entropy change can be obtained in a room temperature range. For example, Gd5(Ge0.5Si0.5)4 shows a maximum entropy change (S) at about 277K and shows an entropy change (S) of about 20 J/(kg·K) when the magnitude of the external magnetic field is changed from 0 to 5 teslas at about 277 K, and shows an entropy change (S) of about 15 J/(kg·K) when the magnitude of the external magnetic field is changed from 0 to 2 teslas. That is, a large entropy change twice or more that of Gd is observed near room temperature. However, to produce large external magnetic field of about 2 to 5 teslas, usually a superconducting magnet must be used. It is impractical to apply a system using a superconducting magnet to ordinary purposes such as refrigeration and air-conditioning.
Furthermore, as a magnetic material capable of obtaining a large entropy change in the range of a magnetic field having a magnitude of, for example, 1 tesla or less, which can be relatively easily produced by permanent magnets in a room temperature range, a lanthanum-iron-silicon La(Fe, Si)13 based intermetallic compound has been proposed (Japanese Patent Application KOKAI No. 2002-356748, F. X. Hu et al., J. Phys. Condens. Matter, 12 (2000), L691; X. X. Zhang et al., Appl. Phys. Lett., Vol. 77, No. 19 (2000), p. 3072; S. Fujieda et al., Appl. Phys. Lett., Vol. 81, No. 7 (2002), p. 1276; and Fujita et al., Materia, Vol. 41, No. 4 (2002), p269).
When these magnetic materials are used as a working substance (hereinafter referred to as a “magnetic refrigeration working substance”) for generating a cycle of temperature in a magnetic refrigeration system, in addition to exhibiting a significantly large temperature difference due to the magnetocaloric effect, it is necessary to exchange heat between the magnetic material and a heat exchange medium effectively. To attain this, these magnetic materials must be processed into a shape having a large specific surface and capable of bringing into sufficient contact with the flow of a heat-exchanging medium. As a specific shape, a honeycomb, which is formed by processing a magnetic material into a thin film and folded like an accordion, a laminate mesh, or spherical particles so as to be packed into a container are considerable.
Furthermore, these magnetic materials are required to have a sufficient mechanical strength for the reasons below. When a cycle of temperature is produced in the magnetic refrigeration system, a magnetic refrigeration working substance is exposed to a flow of a gaseous or liquid heat exchange medium and receives pressure and heat shock. In the case of a brittle magnetic refrigeration working substance, if pressure and heat shock are repeatedly given to the brittle substance, crack and cleavage are produced, generating fine particles. The fine particles thus produced will block the flow channel of a heat-exchanging medium, reducing the performance of the refrigeration system. Particularly, if spherical particles are contained in a magnetic refrigerating chamber, particles collide with each other or strike a chamber wall, generating fine particles. As a result, it may become difficult to maintain a space between particles constant. Consequently, the loss of pressure of the heat exchange medium significantly increases, the performance of the refrigeration system deteriates.
However, gadolinium-germanium-silicon based or lanthanum-iron-silicon based intermetallic compounds are very brittle similarly to most of intermetallic compounds containing rare earth elements and low in mechanical strength compared to Gd metal. Furthermore, the gadolinium-germanium-silicon based and lanthanum-iron-silicon based intermetallic compounds are poor in ductility and malleability similarly to other intermetallic compounds containing rare earth elements. Therefore, mechanical processing of these intermetallic compounds including metal rolling, wire drawing, bending, and shaving is difficult compared to a single metal (such as Cu, Al and Gd) and an alloy material including a Cu based, Fe based and Gd based alloys (such as brass, stainless steel, and permalloy).
Furthermore, since rare earth elements generally have a high chemical activity, the Gd or La containing intermetallic compounds mentioned above are relatively easily oxidized similarly to other intermetallic compounds containing rare earth elements. In particular, the reactivity of the intermetallic compounds with oxygen and nitrogen is high at a high temperature exceeding 1500° C. In other words, the intermetallic compounds react with both oxygen and nitrogen easily. The lighter the molecular weight of a rare earth element, the higher the oxidation activity.
As described above, the gadolinium-germanium-silicon based and lanthanum-iron-silicon based intermetallic compounds are poor in ductility and malleability. Therefore, mechanical processing of such intermetallic compounds into a mesh or sheet is difficult. On the other hand, as a method of forming spherical particles, generally the following methods are known.
(a) A raw material is cut into appropriate pieces, which are then allowed to collide with each other and polished to round into spherical particles;
(b) A raw material is melted in a crucible and the melt is supplied dropwise from a nozzle provided on the tip of the crucible into a sufficiently large gaseous bath (or a liquid bath), to form spherical particles with the help of surface tension and cooled with a gas (or liquid) through heat exchange to solidify the particles (called “atomizing method”);
(c) A raw material is melted in a crucible and the melt is injected on a disk rotating at a high speed. Then, the melt is dropped out from the rotating disk to solidify as particles (called “rotating disk process”);
(d) Broken pieces of a raw material are melted by a plasma jet, sprayed, and solidified into powders (called “plasma-spray method”); and
(e) While an electrode rod formed of a raw material is rotated at a high speed, a current is supplied by a plasma arc discharge. In this manner, the surface of an electrode rod is melt and a melt is simultaneously atomized by centrifugal force and solidified into powder (called “rotating electrode process”).
However, the gadolinium-germanium-silicon based and lanthanum-iron-silicon based intermetallic compounds are very brittle and poor in mechanical strength. Therefore, it is impossible to apply the mechanically spheroidizing process (a) to the compounds.
The intermetallic compounds mentioned above have a high melting temperature of more than 1,500° C., however, the reactivity of the intermetallic compounds to oxygen and nitrogen become extremely high at the melting point or more. Therefore, it is not easy to spheroidize the intermetallic compounds by the atomizing method (b) or rotating disk process (c) using a crucible formed of quartz, alumina, zirconia, BN, or AIN which contain oxygen or nitrogen.
In contrast, the plasma spray method (d) does not employ a crucible and is thus free from the problems mentioned above. In this method, since broken pieces of a raw material are vigorously sprayed together with a plasma jet, the broken pieces are exposed to high temperature only for a short time. This method is therefore suitably used for producing relatively small spherical particles but not suitable for producing relatively large spherical particles. To explain more specifically, to form relatively large particles in diameter, broken pieces are solidified before the pieces are sufficiently melted. As a result, some pieces still keep original shapes or other pieces have corners. Therefore it is difficult to obtain virtually spherical particles. In the aforementioned intermetallic compounds, the plasma spray method (d) is suitable for providing spherical powder of particles having a small diameter of 0.01 mm or less. However, when spherical particles having a relatively large particle diameter (from 0.2 mm to 2 mm) are produced, the ratio of irregular particles increases, conversely virtually spherical particles significantly decreases.
Besides the methods mentioned above, there is a rotating electrode process using no a crucible. In this method, since an electrode rod is rotated at high speed, the material for the electrode rod must have sufficient mechanical strength. More specifically, the electrode rod must be strongly fixed onto a rotation axis during melt. The electrode rod is fixed by a chuck that is widly used in a lathe, or by a screw that is formed at the end of the electrode rod so as to have thread in reverse direction. However, the intermetallic compounds are very brittle and poor in mechanical strength, so that it is difficult to fix the electrode rod made of an intermetallic compound by a chuck with a force sufficient to withstand rapid rotation. In addition, since the intermetallic compounds are poor in ductility and malleability, it is difficult to shave thread into it. Therefore, it is difficult to apply a spheroidizing process using the rotating electrode process (e) to such brittle materials represented by the intermetallic compounds.
As mentioned above, the gadolinium-germanium-silicon based and lanthanum-iron-silicon based intermetallic compounds have an excellent characteristic of a large magnetocaloric effect; however, it has a practical problem: it is not sufficient in mechanical strength for working as a magnetic refrigeration working substance for long period, and it is difficult for such an intermetallic compound to be formed into a shape suitable for a magnetic refrigeration working substance.